In optical networks, a polarization multiplexing technology and a polarization time multiplexing technology are used in order to improve the spectrum utilization efficiency (see, for example, Japanese Laid-open Patent Publication No. 2014-220822). In these technologies, signals are transmitted using optical devices such as optical fibers, multiplexing/demultiplexing devices, optical attenuators, and optical switches.
Firstly, polarization multiplexing is described. The polarization multiplexing modulates a main data signal to two polarization components, in other words, a polarization component State Of Polarization (SOP) 1 and a polarization component SOP2, and multiplexes and transmits the SOP1 and the SOP2 orthogonal to each other. For example, as illustrated in FIG. 16, the polarization multiplexing (PoIMUX) modulates a main data signal to the SOP1 and the SOP2, and inserts the SOP1 and the SOP2 on the time axis and transmits the SOP1 and the SOP2. Meanwhile, the polarization time multiplexing (PolTDM) modulates a main data signal to the SOP1 and the SOP2, and alternately inserts the SOP1 and the SOP2 on the time axis and transmits the SOP1 and the SOP2.
Next, signal degradation caused by the PDL (polarization dependent loss) is described. The PDL refers to a light loss that signal light receives from a transmission line and the like. A received signal is separated into the polarization components SOP1 and SOP2 at a reception side (for example, coherent receiver) of the signal but the signal degradation is caused because the PDL is present in the transmission line.
The first signal degradation is signal degradation due to a power level imbalance. The SOP1 and the SOP2 have different losses generated during the transmission, thereby generating variations in the transmission quality. In other words, as illustrated in FIG. 17, the SOP1 and the SOP2 have the same power level before the transmission (solid line) but have different power levels after the transmission (dashed line) due to the influence of the PDL (a difference is generated). Moreover, it is understood from FIG. 17 that the polarization state on the Poincare sphere is changed between before the transmission and after the transmission due to the transmission. The Poincare sphere represents all the polarization states in one coordinate system, and is used to grasp transition of the polarization states.
The second signal degradation is signal degradation due to a loss of orthogonality. The SOP1 and the SOP2 have the changed polarization states due to the transmission, and lose the orthogonality after the transmission (dashed line) as illustrated in FIG. 18, which results in the generation of crosstalk, in other words, a leakage of the signal. This generates the signal degradation. In the example illustrated in FIG. 18, the above-described level imbalance of the power level is not generated but the orthogonal relation is lost between before and after the transmission (θ2 is not equivalent to θ1). Moreover, it is understood from FIG. 18 that the polarization state on the Poincare sphere is changed between before the transmission and after the transmission due to the transmission.
A method of suppressing the abovementioned signal degradation includes: transmitting some polarization states of the detection signal to the reception side; detecting a polarization state having a best (minimum) bit error rate (BER) at the reception side; and transmitting (feeding back) the result to the transmission side. The transmission side modulates a transmission signal based on the received result (polarization state), and transmits the transmission signal. This makes it possible to suppress the signal degradation.
However, the nonlinear degradation may increase or decrease when, for example, the number of channels dynamically increase or decrease in a transmission line in the course of measuring a detection signal, in other words, the status of the transmission line changes, and thus the result of the BER may become better or worse. In other words, the result of the BER is influenced due to a cause other than the PDL, so that the signal degradation due to the influence of the PDL is failed to be efficiently suppressed in some cases. The nonlinear degradation refers to degradation due to a nonlinear phenomenon (such as self phase modulation) that occurs in the transmission line.
Moreover, the method of suppressing the signal degradation includes a method in which a power level imbalance between two polarization components of a main data signal is determined (power level comparison at the receiver side) based on the power level, and resolves the power level imbalance (see FIG. 19). However, this method causes such a problem that the long converge time is used because the control is requested to perform with the certain steps to optimize the power level imbalance.
Moreover, the method of suppressing the signal degradation includes a method (PDL compensation method) in which the reception side analyzes the distribution (see FIGS. 20A-20D) of reception signals on the Poincare sphere, and returns the distribution to the distribution of transmission signals on the Poincare sphere (see FIGS. 20A-20D), thereby performing polarization multiplexing isolation. FIGS. 20A-20D illustrates a polarization state after X-polarized waves and Y-polarized waves are coupled.
FIG. 20A illustrates a state (polarization state of reception signals) in which the polarization state of transmission signals is rotated due to the transmission. This state is a state that may be returned to the distribution of transmission signals by the rotation at the reception side. FIG. 20B illustrates a state (polarization state of reception signals) in which the polarization state of transmission signals receives an influence of the PDL due to the transmission, and is apart from the center of the Poincare sphere by a distance d. This state is a state of a difference of the power levels before and after the transmission. FIG. 20C illustrates a state (polarization state of reception signals) in which the polarization state of transmission signals receives an influence of the PDL due to the transmission to be apart from the center of the Poincare sphere by a distance D, and is further rotated. This state is a state of a difference of the power levels before and after the transmission. FIG. 20D illustrates a method of returning the reception signal to the distribution of transmission signals on the Poincare sphere. The reception signal is returned to the distribution of transmission signals by the three-step movement of the center this case. Returning to the distribution of transmission signals on the Poincare sphere makes it easy to isolate the polarized and multiplexed signal, and makes it possible to suppress the influence of the PDL.
The PDL compensation method is digital signal processing only at the reception side, which provides no feedback to the transmission side, and the variations in penetrating characteristics of four signal points on the Poincare sphere occurs, thereby generating a difference of the respective optical signal to noise ratios (OSNR). The OSNR is a power ratio between the optical signal and the noise. Therefore, it is impossible to minimize the signal degradation.
Moreover, the methods of suppressing the PDL illustrated in FIG. 19 and FIGS. 20A-20D have such as a problem that an influence of a loss of the orthogonality during the transmission is not considered.
However, signals are subjected to random disturbances due to the polarization variation when the transmission penetrating characteristics have a polarization dependence to cause polarization noise. This polarization noise lowers the transmission quality in optical communications. Therefore, the optical device used in the optical communication, particularly in the Wavelength Division Multiplexing (WDM)/Dense WDM (DWDM) transmissions, is desired to have as small a variation of the transmission loss due to the polarization as possible, that is, as small an influence from Polarization Dependent Loss (PDL) as possible.